The invention relates to the field fill level measurement sensors, and in particular to the field of radar fill level measurement sensors.
To measure the fill level in containers containing for example, liquids or bulk materials, sensors are often employed that utilize the radar principle to measure the propagation time of microwaves traveling from the sensor to the surface of the filling material and back. The microwaves (e.g., in the frequency range between about 1 GHz and 100 GHz) are usually emitted by an antenna as electromagnetic waves to the surface of the filling material, and electromagnetic waves reflected from the surface of the filling material are received at the antenna. Rather than an antenna, a waveguide may be used to conduct the electromagnetic waves to the filling material and back. In this embodiment, the reflection of the waves at the surface of the filling material is based on the change in the propagation impedance of the waveguide for the wave at the filling material surface.
A variety of known radar measuring techniques are used to measure the wave propagation time from the sensor to the measured surface and back. The two main radar measuring techniques are: (i) frequency-modulated continuous wave radar method (FMCW) and (ii) the pulse propagation time method (i.e., pulse radar).
In FMCW systems, the propagation time is measured indirectly by emitting a frequency-modulated signal and determining the difference between the instantaneous emitted and received frequencies. The difference between the signals is proportional to the distance between the sensor and the surface of the material in the container.
Pulse radar systems emit short microwave pulses (i.e., bursts), and determine the time interval between emission and reception of the pulses. The time interval is used to determine the distance between the sensor and filling material surface, and thus the fill level of the container. Since the usual measurement distances are in the range of up to a few meters, the measurement time intervals are extremely short, and as a result with pulse radar sensors the received echo signal usefully undergoes time dilation through a time transformation method. A method of this type is described in U.S. Pat. No. 4,521,778, which discloses a time-dilated echo signal that corresponds to the received high-frequency signal, but proceeds more slowly over time (e.g., by a factor of between 10,000 and 100,000). As a result, a carrier frequency (e.g., 5.8 GHz) for the microwave pulse becomes a carrier frequency of the time-dilated echo pulse of between, for example, 58 kHz and 580 kHz. Within the time-dilated echo signal, the time interval between the transmitted and received pulses may be determined by commonly available circuit components and methods for signal processing and subsequent time measurement, such as those also used by ultrasound fill level sensors.
An additional approach is disclosed in U.S. Pat. No. 6,087,978, wherein the time-dilated echo signal, often also called the intermediate-frequency (IF) signal, is demodulated. The resulting envelope curve signal is sampled and converted to digital values. A digital evaluation then determines the maxima of the transmission pulse and the echo pulse reflected by the measured surface. A disadvantage of this echo signal processing technique based on a linear envelope curve is that when one transmission pulse is reflected by multiple objects and as a result the received echo signal includes a plurality of individual echo pulses, not all of the echo pulses can be simultaneously detected given larger amplitude differences between the different echo pulses received. If the system amplification is adjusted to optimally display large-amplitude echo pulses, the small-amplitude echo pulses are no longer detectable. Conversely, when amplification is adjusted for weak echo pulses, the strong echo pulses are overloaded. This of course significantly impairs the correct evaluation of the echo signals since, of the plurality of pulses within the received echo signal, only one actually comes directly from the measured surface, and a priori one cannot generally say what strength this pulse has relative to other pulses of the echo. Therefore, to identify all possible relevant pulses of an echo signal, it may be necessary to sequentially evaluate a plurality of echoes of different amplitudes.
To avoid this disadvantage, M. Skolnik, Radar Handbook, second edition, McGraw-Hill, pages 3.25 ff, discloses a method in which a logarithmic envelope curve of the IF signal is generated instead of a linear envelope curve. This logarithmic output signal can represent for example a dynamic range of over 80 dB for the echo signal, with the result that very strong and very weak echo pulses may be simultaneously identified and processed within an IF signal by the evaluation unit. This approach makes it possible to acquire all the generated pulses simultaneously, even in containers that due to their characteristics generate spurious echo pulses in addition to the echo pulse from the filling material surface, and to determine which of these pulses is correct.
To improve measurement accuracy, German Patent 44 07 369 describes a method in which both the time difference between the transmission of the sampled signal and reception of the echo pulse is measured and evaluated, in addition to the phase difference between the sampled signal and the echo pulse. As a result, the envelope curve of the echo signal is generated in order to measure the time difference, while the IF signal is amplified in a parallel signal circuit and evaluated in terms of the relative phase position of the transmitted sampled signal and the received echo. While this approach achieves extremely precise measurement accuracy, the required complexity of the signal processing electronics is considerable. For example, the additional signal processing circuit for phase evaluation contains a quadrature demodulator and, at least one analog-digital converter (ADC) for sampling the I-Q output of the quadrature demodulator.
An alternative approach would be to sample the IF signal directly with a fast ADC and to perform the quadrature demodulation, and thus the phase evaluation in the discrete time domain. However, this approach also requires a relatively complex circuit due to the high amplitude dynamics of this signal, which requires an adjustable amplifier prior to the digitization.
Another approach to recovering the phase information from the echo of the IF signal is to use a limiter as described in the above-referenced book by Skolnik starting on page 3.30. The edges of the approximately rectangular output signal of the limiter mark the zero crossings of the echo pulse carrier. They may be used to trigger timers to determine the phase position of these echoes. Since use of the limiter obtains phase information at the expense of amplitude information, determination of the phase information is possible only in an additional parallel signal circuit for envelope curve processing which of course involves additional circuit complexity and cost.
Known prior art techniques for signal-processing an echo signal in a fill level system to measure the distance of an object either: (i) forego the increased measurement accuracy achievable by evaluation of the phase by limiting the approach to evaluating the envelope curve, or (ii) significantly increase the circuit complexity relative to a xe2x80x9csimplexe2x80x9d propagation-time-evaluation approach by providing a complex parallel signal processing circuit to extract phase information.
Therefore, there is a need for an improved signal processing technique in a pulse radar fill level sensor for measuring the distance to an object that provides fast detection of echo pulses of highly differentiated intensities, as well as extraction of the phase information for different echo pulses from the echo signal.
Briefly, according to an aspect of the invention, a radar fill level measurement sensor for measuring the level of material in a container includes a transmit/receive circuit that transmits an electromagnetic pulse, and detects reflected electromagnetic pulses and provides an intermediate frequency signal indicative of the reflected electromagnetic pulses. A logarithm detector circuit receives the intermediate frequency signal and provides a logarithmic output signal indicative of the logarithm of the intermediate frequency signal. An evaluation unit receives and processes the logarithmic output signal, to determine the fill level of the material within the container, and provides a fill level measurement signal indicative thereof.
By logarithmizing the received echo signal, the dynamic range in which a received echo pulse is identifiable and evaluatable is enhanced. The system measures a phase difference between the transmitted pulse-shaped signal and the echo pulse, and the time difference between these in the logarithmized echo signal.
Since the logarithmic output signal cannot be negative, it is not possible to use zero crossings to measure the phase difference. Instead the minima of the logarithmized echo are determined, preferably by differentiation.
In one embodiment, the received echo signal is input to a logarithmic detector circuit that includes a plurality of cascaded saturable amplifiers, wherein the output signals of the individual saturable amplifiers are summed to obtain the logarithm, then fed to an input of a following amplifier of the cascade as the input signal. The output of the last amplifier of this cascade supplies a signal that represents the echo signal that has been limited in its amplitude by saturation and amplified by the product of the gains of all of the cascade""s amplifiers. Due to this high amplification outside the saturation range, this output signal contains nearly rectangular oscillation patterns such that their zero crossing points are an exact image of the zero crossings of the detected echo.
To measure the time difference between a transmitted signal and a desired pulse of the logarithmic echo signal, in one embodiment the logarithmic output signal is low-pass filtered to recover the envelope of the echo signal.
A method according to an aspect of the invention, which may be employed preferably in two designs based on the following description, uses the following common approach: extraction of a time difference signal between a reference pulse and an echo pulse of a pulse-propagation-time measurement, and correction of the time difference determined through measurement of the phase difference, where the time difference measurement and phase difference measurement share as many circuit components as possible.
In one embodiment, the logarithmized echo signal may be employed as the starting point for the time difference measurement, while the phase difference measurement is based on the limited echo signal also obtained during logarithmation.
These and other objects, features and advantages of the present invention will become more apparent in light of the following detailed description of preferred embodiments thereof, as illustrated in the accompanying drawings.